It has been recognized that if integrated optical circuits are to be incorporated into fiber communication systems, a practical method of coupling such circuits to glass fibers must be devised.
The typical integrated optical device is fabricated by the deposition of doped silica films on a silicon substrate. The films are then patterned to produce waveguides. See, for example, J. T. Boyd et at., Optical Engineering Vol. 24, No. 2, pp. 230-234, (1958), and F. S. Hickernell, Solid State Technology, Vol. 31, No. 11, pp. 83-88, 1988. The use of such techniques permits the fabrication of a variety of integrated optical devices such as multiplexers, adiabatic polarization splitters, and array star couplers.
In order to couple into and out of such devices, optical fibers are butt-coupled to the waveguide ends, and secured thereto by means of a bonding material such as an epoxy. In order to stabilize the attached array of fibers, and to provide sufficient confinement of the evanescent field of the guided wave, the device may be covered with a thin silica coverplate, affixed with a low loss adhesive, as described in U.S. Pat. No. 5,076,654, issued Dec. 31, 1991, and assigned to applicant's assigne.
Methods for aligning and permanently attaching fibers to waveguide devices on lithium niobate are described in a paper entitled "Fiber Attachment for Guided Wave Devices" by E. J. Murphy, published in the Journal of Lightwave Technology, Vol. 6, No. 6, June 1988. As indicated in this paper, permanent attachment, after alignment, is typically made using some type of adhesive.
The problem with the above-described arrangements is that the use of an epoxy, or other type or adhesive in the optical wavepath, is unacceptable for certain applications, such as submarine cables. Heat, applied during the curing period, or as a result of high intensity light signals, tends to deteriorate the adhesive and, hence, the bond. Accordingly, the use of such adhesives is to be avoided.
An alternative bonding technique, using glass as the adhesive, as described in U.S. Pat. No. 5,208,885, issued May 4, 1993 and assigned to applicant's assigne, produces very satisfactory heat insensitive connections. However, it has a number of commercial drawbacks. For example, the implementation of this technique requires a number of additional processing steps such as: the formation of a low melting temperature glass film on the end of each waveguide (or fiber); and the polishing of such films to make them flat. In addition, the film, being of a different material than the fiber and waveguide, may introduce stresses and strains in the optical wavepath at the waveguide-fiber junction.
It is, accordingly, the object of the present invention to avoid the use of bonding materials by providing a new waveguiding structure, and a method of fusing such a structure directly to an optical fiber using currently available technology.